Aneurysms occur in blood vessels at sites where, due to age, disease or genetic predisposition of the patient, the strength or resilience of the vessel wall is insufficient to prevent ballooning or stretching of the wall as blood passes through. If the aneurysm is left untreated, the blood vessel wall may expand and rupture, often resulting in death.
Stent grafts may be used to treat aneurysms. A stent graft includes a graft material secured to a cylindrical scaffolding or framework of one or more stents. The stent graft may be introduced into a blood vessel percutaneously and deployed to span the aneurysmal sac. The stent(s) provide rigidity and structure to hold the graft material open in a tubular configuration as well as the outward radial force needed to create a seal between the graft material and a healthy portion of the vessel wall. Blood flowing through the vessel can be channeled through the hollow interior of the stent graft to reduce or eliminate the stress on the vessel wall at the location of the aneurysmal sac.
Aneurysms occurring in the aorta, the largest artery in the human body, may occur in the chest (thoracic aortic aneurysm) or in the abdomen (abdominal aortic aneurysm). With the advent of fenestration, branch and chimney techniques, it has been possible to treat patients with short angulated necks, aneurysmal extension into either internal iliac arteries or complex aneurysmal involvement of the juxtarenal, paravisceral, and thoracoabdominal aorta using minimally invasive techniques (e.g., EVAR or f-EVAR). The techniques may involve the use of a modular stent graft having (a) a main body stent graft to cover and achieve patency in the aneurysmatic aorta and (b) a side arm stent graft to reach target vessels (e.g., celiac, SMA, renal arteries, internal iliacs, great vessels of the aortic arch). The target vessels exhibit varying levels of tortuousity, with acute tortuousity shown by great vessels of the aortic arch and iliac branches.
The stent used to bridge the gap between the main graft and the target vessel has to endure branch vessel motion relative to the aorta, arterial motion, and motion due to respiration and physical movements which may cause it to crush or collapse at the interface. This warrants the design of more radially stiff bridging stents without sacrificing the plasticity needed to deform and anchor such stents in place. However, the bridging stents also encounter branch vessel tortuousity, which makes it necessary for the stents to be flexible. Flexibility and radial stiffness are antipodal and it is generally believed that radial stiffness must be sacrificed to achieve flexibility and vice versa.